Some time ago I've increased LUN containing single extent of VMFS datastore (ESX 4.1, VMFS 3.47), and tried to extend ("expand") the datastore to use all new capacity. However, when I clicked "Increase..." button in datastore properties window, I found that I can only add new extent, but not to expand the existing one: "Expandable" column for my LUN was "No" despite the fact that there are now several gigs of free space on the LUN after the datastore. Adding new extent on a free space was possible, but I didn't want to do that. After searching on support forums, I've found other discussions where people had similar problems, but no solution.
As an experiment, I've created another datastore on another LUN smaller than containing LUN size, and that was fine - I could expand it's extent without problems. After compairing paritition structure of both LUNs, I've realized that single extent of second (experimental) datastore was a primary partition, while non-expandable extent of first datastore was a logical drive inside of extended partition. An obvious guess: may be for extent to be expandable, it must be primary partition? To test it, I've converted extent of first datastore from logical drive to primary partition (by directly editing partition table with old good Norton Disk Editor), and voila - "Expandable" field changed to "Yes", and I was able to successfully expand the extent to full LUN capacity.
I didn't see any information in documentation regarding this limitation, so somebody may find this information useful.